Effects of hypoxia and reoxygenation on nitric oxide production and cerebral blood flow in developing rat striatum

A visualization system for erectile vascular dynamics

Erection is an essential process which requires the male penis for copulation. This copulatory process depends on the vascular dynamic regulation of the penis. The corpus cavernosum (CC) in the upper (dorsal) part of the penis plays a major role in regulating blood flow inside the penis. When the CC is filled with blood, the sinusoids, including micro-vessels, dilate during erection. The CC is an androgen-dependent organ, and various genital abnormalities including erectile dysfunction (ED) are widely known. Previous studies have shown that androgen deprivation by castration results in significantly decreased smooth muscles of the CC. Experimental works in erectile biology have previously measured intracavernosal penile pressure and mechanical tension. Such reports analyze limited features without assessing the dynamic aspects of the erectile process. In the current study, we established a novel explant system enabling direct visual imaging of the sinusoidal lumen to evaluate the dynamic movement of the cavernous space. To analyze the alternation of sinusoidal spaces, micro-dissected CC explants by patent blue dye injection were incubated and examined for their structural alternations during relaxation/contraction. The dynamic process of relaxation/contraction was analyzed with various external factors administered to the CC. The system enabled the imaging of relaxation/contraction of the lumens of the sinusoids and the collagen-containing tissues. Histological analysis on the explant system also showed the relaxation/contraction. Thus, the system mimics the regulatory process of dynamic relaxation/contraction in the erectile response. The current system also enabled evaluating the erectile pathophysiology. In the current study, the lumen of sinusoids relaxed/contracted in castrated mice similarly with normal mice. These results suggested that the dynamic erectile relaxation/contraction process was similarly retained in castrated mice. However, the system also revealed decreased duration time of erection in castrated mice. The current study is expected to promote further understanding of the pathophysiology of ED, which will be useful for new treatments in the future. Hence, the current system provides unique information to investigate the novel regulations of erectile function, which can provide tools for analyzing the pathology of ED.

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Regional cerebral blood flow in natives at high altitude: An arterial spin labeled MRI study

BACKGROUND

It is known that a neurologic sequence occurs at high altitudes (HA); hence, cerebral blood flow (CBF) might vary by altitude.

PURPOSE

To use arterial spin labeled (ASL) MRI to evaluate absolute CBF differences between subjects who live at HA and lowlands.

STUDY TYPE

Cohort prospective trial.

POPULATION

In all, 64 HA Tibetans, 19 lowland Tibetans, and 25 lowland Han subjects.

FIELD STRENGTH/SEQUENCE

CBF was measured with the pulsed ASL sequence at 3T.

ASSESSMENT

CBF was correlated with abode altitude in HA Tibetans; CBF differences among HA Tibetans, lowland Tibetans, and lowland Han subjects was assessed.

STATISTICAL TESTS

Pearson correlation assessed the correlation. Independent t-tests analyzed group differences.

RESULTS

In HA Tibetans, CBF decreased with altitude in the bilateral anterior and posterior cingulate gyri, fusiform gyrus, cerebellar tonsil and cortices, and thalamus as well as left middle and inferior temporal gyri and right insula (P < 0.05); HA Tibetans (vs. lowland Tibetans) had lower CBF in the left hemisphere (precuneus, anterior cingulate gyrus, fusiform gyrus, and lingual gyrus) and right hemisphere (superior parietal lobule, precuneus, posterior cingulate gyrus, and cerebellar tonsil), while they had higher CBF in the left inferior parietal lobule, lentiform nucleus, and inferior frontal gyrus (P < 0.05). The overlapping regions, in which CBF in HA Tibetans correlated with altitude and decreased (vs. lowland Tibetans), were selected for region of interest analysis, and the results showed lower CBF in HA Tibetans than lowland Han subjects (P < 0.05).

DATA CONCLUSION

HA adaptation in Tibetans is associated with a decrease of regional CBF.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 4 J. Magn. Reson. Imaging 2018.

Granulocyte-Colony Stimulating Factor Increases Cerebral Blood Flow via a NO Surge Mediated by Akt/eNOS Pathway to Reduce Ischemic Injury

Granulocyte-colony stimulating factor (G-CSF) protects brain from ischemic/reperfusion (I/R) injury, and inhibition of nitric oxide (NO) synthases partially reduces G-CSF protection. We thus further investigated the effects of G-CSF on ischemia-induced NO production and its consequence on regional cerebral blood flow (rCBF) and neurological deficit. Endothelin-1 (ET-1) microinfused above middle cerebral artery caused a rapid reduction of rCBF (ischemia) which lasted for 30 minutes and was followed by a gradual recovery of blood flow (reperfusion) within the striatal region. Regional NO concentration increased rapidly (NO surge) during ischemia and recovered soon to the baseline. G-CSF increased rCBF resulting in shorter ischemic duration and an earlier onset of reperfusion. The enhancement of the ischemia-induced NO by G-CSF accompanied by elevation of phospho-Akt and phospho-eNOS was noted, suggesting an activation of Akt/eNOS. I/R-induced infarct volume and neurological deficits were also reduced by G-CSF treatment. Inhibition of NO synthesis by L-N^G-Nitroarginine Methyl Ester (L-NAME) significantly reduced the effects of G-CSF on rCBF, NO surge, infarct volume, and neurological deficits. We conclude that G-CSF increases rCBF through a NO surge mediated by Akt/eNOS, which partially contributes to the beneficial effect of G-CSF on brain I/R injury.

Ischemic postconditioning in cerebral ischemia: Differences between the immature and mature brain?

Ischemic postconditioning (postC), defined as serial mechanical interruptions of blood flow at reperfusion, effectively reduces myocardial infarct size in all species tested so far, including humans. In the brain, ischemic postC leads to controversial results regardless of variations in factors such as onset time of beginning, the duration of ischemia and/or reperfusion, and the number of cycles of occlusion/reperfusion. Thus, many major issues remain to be resolved regarding its protective effects. Future studies should aim to identify the parameters that yield the strongest protection, as well as to understand why the efficacy of ischemic postC differs between models. This review will focus on initial hemodynamic changes and their consequences, and on specific features such as NO-dependent vascular tone and/or prolonged acidosis in cerebral ischemia-reperfusion in order to better understand the dynamics of ischemic postC in the developing brain.

Dynamic spatio-temporal imaging of early reflow in a neonatal rat stroke model

The aim of the study was to better understand blood-flow changes in large arteries and microvessels during the first 15 minutes of reflow in a P7 rat model of arterial occlusion. Blood-flow changes were monitored by using ultrasound imaging with sequential Doppler recordings in internal carotid arteries (ICAs) and basilar trunk. Relative cerebral blood flow (rCBF) changes were obtained by using laser speckle Doppler monitoring. Tissue perfusion was measured with [(14)C]-iodoantipyrine autoradiography. Cerebral energy metabolism was evaluated by mitochondrial oxygen consumption. Gradual increase in mean blood-flow velocities illustrated a gradual perfusion during early reflow in both ICAs. On ischemia, the middle cerebral artery (MCA) territory presented a residual perfusion, whereas the caudal territory remained normally perfused. On reflow, speckle images showed a caudorostral propagation of reperfusion through anastomotic connections, and a reduced perfusion in the MCA territory. Autoradiography highlighted the caudorostral gradient, and persistent perfusion in ventral and medial regions. These blood-flow changes were accompanied by mitochondrial respiration impairment in the ipsilateral cortex. Collectively, these data indicate the presence of a primary collateral pathway through the circle of Willis, providing an immediate diversion of blood flow toward ischemic regions, and secondary efficient cortical anastomoses in the immature rat brain.

Cerebral blood flow during reperfusion predicts later brain damage in a mouse and a rat model of neonatal hypoxic-ischemic encephalopathy

Children with severe neonatal hypoxic-ischemic encephalopathy (HIE) die or develop life-long neurological impairments such as cerebral palsy and mental retardation. Decreased regional cerebral blood flow (CBF) is believed to be the predominant factor that determines the level of tissue injury in the immature brain. However, the spatio-temporal profiles of CBF after neonatal HIE are not well understood. CB17 mouse and Wistar rat pups were exposed to a unilateral hypoxic-ischemic (HI) insult at eight or seven days of age. Laser speckle imaging sequentially measured the cortical surface CBF before the hypoxic exposure and until 24h after the hypoxic exposure. Seven days after the HI insult, brain damage was morphologically assessed by measuring the hemispheric volumes and by semi-quantitative scoring for neuropathologic injury. The mean CBF on the ipsilateral hemisphere in mice decreased after carotid artery ligation. After the end of hypoxic insult (i.e., the reperfusion phase), the mean CBF level gradually rose and nearly attained its pre-surgery level by 9h of reperfusion. It then decreased. The degree of reduced CBF during reperfusion was well correlated with the degree of later morphological brain damage. The correlation was the strongest when the CBF was measured in the ischemic core region at 24h of reperfusion in mice (R²=0.89). A similar trend in results was found in rats. These results suggest that the CBF level during reperfusion may be a useful predictive factor for later brain damage in immature mice. This may enable optimizing brain damage for detail analyses.

Perivascular nitric oxide and superoxide in neonatal cerebral hypoxia-ischemia

Decreased cerebral blood flow (CBF) has been observed following the resuscitation from neonatal hypoxic-ischemic injury, but its mechanism is not known. We address the hypothesis that reduced CBF is due to a change in nitric oxide (NO) and superoxide anion O(2)(-) balance secondary to endothelial NO synthase (eNOS) uncoupling with vascular injury. Wistar rats (7 day old) were subjected to cerebral hypoxia-ischemia by unilateral carotid occlusion under isoflurane anesthesia followed by hypoxia with hyperoxic or normoxic resuscitation. Expired CO(2) was determined during the period of hyperoxic or normoxic resuscitation. Laser-Doppler flowmetry was used with isoflurane anesthesia to monitor CBF, and cerebral perivascular NO and O(2)(-) were determined using fluorescent dyes with fluorescence microscopy. The effect of tetrahydrobiopterin supplementation on each of these measurements and the effect of apocynin and N(omega)-nitro-L-arginine methyl ester (L-NAME) administration on NO and O(2)(-) were determined. As a result, CBF in the ischemic cortex declined following the onset of resuscitation with 100% O(2) (hyperoxic resuscitation) but not room air (normoxic resuscitation). Expired CO(2) was decreased at the onset of resuscitation, but recovery was the same in normoxic and hyperoxic resuscitated groups. Perivascular NO-induced fluorescence intensity declined, and O(2)(-)-induced fluorescence increased in the ischemic cortex after hyperoxic resuscitation up to 24 h postischemia. L-NAME treatment reduced O(2)(-) relative to the nonischemic cortex. Apocynin treatment increased NO and reduced O(2)(-) relative to the nonischemic cortex. The administration of tetrahydrobiopterin following the injury increased perivascular NO, reduced perivascular O(2)(-), and increased CBF during hyperoxic resuscitation. These results demonstrate that reduced CBF follows hyperoxic resuscitation but not normoxic resuscitation after neonatal hypoxic-ischemic injury, accompanied by a reduction in perivascular production of NO and an increase in O(2)(-). The finding that tetrahydrobiopterin, apocynin, and L-NAME normalized radical production suggests that the uncoupling of perivascular NOS, probably eNOS, due to acquired relative tetrahydrobiopterin deficiency occurs after neonatal hypoxic-ischemic brain injury. It appears that both NOS uncoupling and the activation of NADPH oxidase participate in the changes of reactive oxygen concentrations seen in cerebral hypoxic-ischemic injury.

Exercise and food ad libitum reduce the impact of early in life nutritional inbalances on nitrergic activity of hippocampus and striatum

Nutritional imbalances were produced by varying litter size pups per dam: 3 (small), 6 (medium), and 12 (large). On the 21st day, 4 subjects of each litter, were sacrificed and the remaining were grouped, 2 per cage, with or without running wheels, with food and water ad libitum. Adult subjects were tested in water maze, their brains processed for NADPH-diaphorase histochemistry and quantified by densitometry. No differences were detected in water maze. At 21st day, S and L compared with M presented reduced NADPH-d in the stratum molecular of dentate gyrus (DG), stratum lacunosum of CA1 and in all CA3 layers but not in the striatum. On the 58th day, actvity remained low in S and L in CA3 and striatum and L in CA1 and DG. Voluntary exercise increased NADPH-d in DG, CA1, CA3, and striatum in S, and in the stratum lacunosum of CA1 and CA3 in L.

Nitrotyrosine in human neonatal spinal cord after perinatal asphyxia

BACKGROUND

Spinal cord injury has been reported after perinatal asphyxia in full-term neonates.

OBJECTIVES

To examine the role of excessive nitric oxide production in perinatal spinal cord injury.

SUBJECTS AND METHODS

Tissue samples of 18 full-term neonates who died of hypoxic-ischemic encephalopathy were analyzed for the presence of nitrotyrosine (NT).

RESULTS

NT was demonstrated in 5 of these 18 neonates. In addition, activated caspase 3, a marker of apoptosis, and CD68, as a marker of inflammation, could be demonstrated in some infants.

CONCLUSIONS

excessive nitric oxide production and subsequent NT formation is seen in spinal cord tissue after severe perinatal asphyxia. This finding may be relevant for the development of neuroprotective strategies.

The effect of melatonin on protein oxidation and nitric oxide in the brain tissue of hypoxic neonatal rats

Melatonin is a potent antioxidant agent that can scavenge oxy- and nitroradicals generated under hypoxic conditions in the brain. In this study, we investigated the effect of melatonin on protein oxidation and nitric oxide (NO) during hypoxia. Seven-day-old Sprague-Dawley newborn rats were divided into three groups. Hypoxic (n=9) and melatonin (n=11) groups were subjected to 2h of hypoxic exposure (a humidity mixture of gases consisting of 92% nitrogen and 8% oxygen). Melatonin (at a dose of 10mg/kg) was administrated 30 min before the onset hypoxia and then at 24th and 48th hours after the end of the hypoxic exposure. Control (n=10) and hypoxic groups received the isotonic sodium chloride according to the same schedule. The brain tissue concentration of advanced oxidation protein products (AOPP) and protein thiol (P-SH) was used as an index of protein oxidation. In our study, although AOPP and NO increased significantly, the levels of P-SH decreased in the hypoxic group. The level of AOPP was declined by melatonin treatment. However, perturbed thiol status could not be recovered by melatonin treatment. There was no relationship between the levels of NO and protein oxidation markers. These results indicate that exogenous melatonin could prevent AOPP, but that it is inadequate in recovering perturbed thiol status. Therefore, melatonin alone was observed to be an incomplete treatment to prevent protein oxidation in hypoxia-induced brain damage.

Nitrotyrosine in brain tissue of neonates after perinatal asphyxia

HYPOTHESIS

Nitrotyrosine, a reaction product of peroxynitrite and proteins, could be demonstrated in the postmortem examination of brain tissue of full-term neonates who had severe perinatal asphyxia.

METHODS

The brain tissue of 22 full-term neonates who died after severe perinatal asphyxia was examined, including cerebral cortex, basal ganglia, thalamus, hippocampus, brain stem, olives and cerebellum. Median age at death was 52 h. Routine histopathological examination and additional immunohistological staining were carried out with anti-cysteine protease protein 32 antibodies to detect activated caspase 3, anti-nitrotyrosine antibodies to detect nitrotyrosine and anti-CD68 antibodies to detect activated microglia and macrophages, which might be associated with the production of nitric oxide. Staining was scored as none, weak (1-25% positive cells), moderate (26-75% positive cells) or severe (>75% positive cells).

RESULTS

14 patients showed global injury, 4 showed injury of the basal ganglia and thalamus, and 4 showed predominantly parasagittal brain injury. One neonate without perinatal asphyxia served as a control. Nitrotyrosine staining of neurones was shown in all neonates with asphyxia, mostly in the thalamus (70%) and inferior olives (68%). Total nitrotyrosine staining tended to be less in the base of the pons and inferior olives of neonates with parasagittal brain injury. Activated caspase 3 was found mostly in the thalamus (60%) and hippocampus (53%). Positive CD68 staining was mainly present in the thalamus (70% positive).

CONCLUSION

Nitrotyrosine was found in brain tissue of full-term neonates, suggesting that nitric oxide toxicity might have a role in hypoxic-ischaemic brain injury at term. This may be relevant for neuroprotective strategies in full-term neonates with perinatal asphyxia.

Physiologically relevant measurements of nitric oxide in cardiovascular research using electrochemical microsensors

Nitric oxide (NO) plays an important role in the regulation of blood flow. Pharmacological tools and a series of other techniques have been developed for studying the NO/L-arginine pathway, but it has proved difficult to make a quantitative link between effect and tissue NO concentration. NO microsensors have been applied with success for the measurement of NO in suspensions of mitochondria and cells, such as platelets and leukocytes, and in cell cultures, which together with other interventions or measurements are particularly useful for the examination of cell signalling related to the NO/L-arginine pathway. In isolated vascular segments, studies using the NO microsensor have defined the relationship between NO concentration and relaxation and revealed residual NO release in the presence of NO synthase inhibitors. Moreover, simultaneous measurements of NO concentration and vasorelaxation in isometric preparations have shown that agonist-induced relaxation is L-arginine dependent and NO release is reduced in hypertension. By placing NO microsensors in catheters, it is possible to measure NO in the living animal and man. This approach has been applied for the measurements of NO concentration in relation to increases in flow, erection, in conditions of hypoxia, and in endotoxemia. However, further methodological development of NO microsensors is necessary to avoid the influence of changes in temperature, pH and oxygen on the measurements.

Nitric oxide mediates hypoxia-induced changes in paracellular permeability of cerebral microvasculature

Ischemic stroke from a reduction in blood flow to the brain microvasculature results in a subsequent decreased delivery of oxygen (i.e., hypoxia) and vital nutrients to endothelial, neuronal, and glial cells. Hypoxia associated with stroke has been shown to increase paracellular permeability of the blood-brain barrier, leading to the release of cellular mediators and brain tissue injury. Whereas reperfusion does not occur in all ischemic strokes, increased permeability has been seen in posthypoxic reoxygenation. Currently, it is unknown whether these deleterious effects result from cellular mechanisms stimulated by decreased oxygen during stroke or posthypoxic reoxygenation stress. This study used primary bovine brain microvessel endothelial cells (BBMECs) to examine the involvement of nitric oxide (NO) as a mediator in hypoxia-induced permeability changes. Hypoxia-induced increased transport of [14C]sucrose across BBMEC monolayers compared with normoxia was attenuated by either posthypoxic reoxygenation or inhibition of NO synthase (NOS). The hypoxia-induced permeability effect was further reduced when NOS inhibition was combined with posthypoxic reoxygenation. Additionally, a significant increase in total NO was seen in BBMECs after hypoxic exposure. This correlation was supported by the increased [14C]sucrose permeability observed when BBMECs were exposed to the NO donor diethylenetriaamine NONOate. Western blot analyses of NOS isoforms showed a significant increase in the inducible isoform after hypoxic exposure with a subsequent reduction in expression on reoxygenation. Results from this study suggest that hypoxia-induced blood-brain barrier breakdown can be diminished by inhibition of NO synthesis, decreased concentration of NO metabolites, and/or reoxygenation.

Role of nitric oxide in hypoxic cerebral vasodilatation in the ovine fetus

To investigate the role of nitric oxide (NO) in fetal cerebral circulatory responses to acute hypoxia, near-term fetal sheep were instrumented with laser Doppler probes placed in the parasagittal parietal cortices and vascular catheters in the sagittal sinus and brachiocephalic artery. After a 3 day recovery period, responses of cortical blood flow (CBF) to hypoxia were compared with and without inhibition of nitric oxide synthase (NOS). After an initial 30 min baseline period, fetuses were given a bolus followed by a continuous infusion of Nomega-nitro-L-arginine methyl ester (L-NAME), or saline vehicle as control. After administration of L-NAME, CBF decreased by 14 +/- 6 % (P < 0.01) despite increases in arterial blood pressure of 15 mmHg, resulting in an ~60 % increase in cerebrovascular resistance. Thirty minutes following initiation of L-NAME or vehicle infusion, fetal systemic hypoxia was induced by allowing the ewes to breathe 10-11 % oxygen. In control fetuses CBF increased progressively to 145 +/- 9 % of baseline (P < 0.01) after 30 min, while cortical release of cyclic guanylate cyclase (cGMP), an index of NOS activity, increased 26 +/- 8 % (P < 0.05). In contrast, CBF in L-NAME-treated fetuses increased to only 115 % of the reduced CBF baseline, whereas cortical release of cGMP did not change significantly. In summary, basal levels of NO lower resting cortical vascular resistance by ~15 % in the fetal sheep. Inhibition of NO synthesis attenuates hypoxic cerebral relaxation but does not completely prevent the characteristic increases in CBF. Hypoxic increases in NO directly increase cortical production of cGMP and inhibition of NO synthesis ablates these changes in cGMP.

Potentiation of intracellular Ca2+ mobilization by hypoxia-induced NO generation in rat brain striatal slices and human astrocytoma U-373 MG cells and its involvement in tissue damage

The relationship between nitric oxide (NO) and intracellular Ca2+ in hypoxic-ischemic brain damage is not known in detail. Here we used rat striatal slices perfused under low-oxygen and Ca2+-free conditions and cultured human astrocytoma cells incubated under similar conditions as models to study the dynamics of intracellular NO and Ca2+ in hypoxia-induced tissue damage. Exposure of rat striatal slices for 70 min to low oxygen tension elicited a delayed and sustained increase in the release of 45Ca2+. This was potentiated by the NO donors sodium nitroprusside (SNP) and spermine-NO and inhibited by N-omega-nitro-L-arginine methyl ester (L-NAME) or by the NO scavenger 2-phenyl-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide (PTIO). A membrane-permeant form of heparin in combination with either ruthenium red (RR) or ryanodine (RY) also inhibited 45Ca2+ release. In human astrocytoma U-373 MG cells, hypoxia increased intracellular Ca2+ concentration ([Ca2+]i) by 67.2 +/- 13.1% compared to normoxic controls and this effect was inhibited by L-NAME, PTIO or heparin plus RR. In striatal tissue, hypoxia increased NO production and LDH release and both effects were antagonized by L-NAME. Although heparin plus RR or RY antagonized hypoxia-induced increase in LDH release they failed to counteract increased NO production. These data therefore indicate that NO contributes to hypoxic damage through increased intracellular Ca2+ mobilization from endoplasmic reticulum and suggest that the NO-Ca2+ signalling might be a potential therapeutic target in hypoxia-induced neuronal degeneration.

Fetal lamb cerebral blood flow (CBF) and oxygen tensions during hypoxia: a comparison of laser Doppler and microsphere measurements of CBF

This study was undertaken to compare microsphere and laser Doppler flowmetry techniques for the measurement of cerebral blood flow, to assess the effect of probe implantation at the tip of the sensing probe and to measure brain tissue P(O2) (tP(O2)) in response to acute hypoxia. Fetal sheep of ~131 days gestation (n = 8) were chronically instrumented with bilateral laser Doppler probes in the parietal cortices and catheters for injection of fluorescent microspheres. Five days after surgery fetuses were subjected to 1 h periods of baseline control breathing, hypoxia and recovery. Microspheres were injected 10 min prior to and 10, 30, 50 and 120 min after initiation of hypoxia. Microspheres were counted in four 12 mm(3) tissue samples from each hemisphere, the tip of the laser Doppler probe being positioned in the centre of one of the cubes. The cube containing the probe tip was also subdivided into 4 mm(3) pieces of tissue. In response to hypoxia, fetal arterial P(O2) declined from 21 +/- 2 to 12 +/- 1 Torr and brain tissue P(O2) fell from 10 +/- 1 to a nadir of 1 +/- 1 Torr. Each method detected a significant increase in CBF that reached a maximum after 30-45 min, although the increase of flow measured by laser Doppler flowmetry was less than that measured by spheres after 10 and 30 min (P < 0.05). Microspheres did not detect altered flow at the probe tip or heterogeneity of flow in surrounding volumes of cortical tissue. In summary, laser Doppler flowmetry is a useful measure of continuous relative changes of CBF in the chronically instrumented fetal sheep. Flow compensations in acute hypoxia are not adequate to sustain O(2) delivery, and other compensations, including reduced metabolic rate, are possible.

Fetal cerebral and peripheral circulatory responses to hypoxia after nitric oxide synthase inhibition

The increase in cerebral blood flow (CBF) during hypoxia in fetal sheep at 0.6 gestation is less than the increase at 0.9 gestation when normalized for differences in baseline CBF and oxygen consumption. Nitric oxide (NO) synthase (NOS) catalytic activity increases threefold during this period of development. We tested the hypothesis that administration of the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) decreases the CBF response to systemic hypoxia selectively at 0.9 gestation. We also tested whether any peripheral vasoconstriction during hypoxia is potentiated by L-NAME at 0.9 gestation. Administration of L-NAME increased arterial blood pressure and decreased microsphere-determined CBF during normoxia in fetal sheep at both 0.6 and 0.9 gestation. With subsequent reduction of arterial oxygen content by approximately 50%, the percent increase in forebrain CBF in a control group (57 +/- 11%; +/- SE) and L-NAME-treated group (51 +/- 6%) was similar at 0.6 gestation. Likewise, at 0.9 gestation, the increase in CBF was similar in control (90 +/- 25%) and L-NAME (80 +/- 28%) groups. At 0.9 gestation, L-NAME treatment attenuated the increase in coronary blood flow and increased gastrointestinal vascular resistance during hypoxia. We conclude that NO exerts a basal vasodilatory influence in brain as early as 0.6 gestation in fetal sheep but is not an important mechanism for hypoxic vasodilation in brain at either 0.6 or 0.9 gestation. Thus the developmental increase in NOS catalytic capacity does not appear to be responsible for developmental increases in the CBF response to hypoxia during this period. In contrast, NO modulates the vascular response to hypoxia in heart and gastrointestinal tract.

Prolonged suppression of brain nitric oxide synthase activity by 7-nitroindazole protects against cerebral hypoxic-ischemic injury in neonatal rat

Nitric oxide mediates glutamate-induced excitotoxicity associated with cerebral hypoxia-ischemia through production in the brain by several isoforms of nitric oxide synthase (NOS). We examined the influence of the selective neuronal NOS inhibitor, 7-nitroindazole (7-NI), on brain NOS activity and its neuroprotective effects against cerebral hypoxic-ischemic injury in the postnatal day (PND) 7 rat. In the first set of experiments, 7-NI (50 mg/kg) administered intraperitoneally (i.p.) transiently inhibited NOS activity to 40% below the vehicle control level at 1 h after injection (P<0.001, analysis of variance (ANOVA)). In contrast, 7-NI (100 mg/kg, i.p.) inhibited NOS activity to 56% below the control level at 1 h with prolonged suppression of NOS activity at 3, 6, 9 and 12 h after injection. Two-factor ANOVA revealed an overall effect on NOS activity of 7-NI treatment (P<0.001) and time after injection (P<0.001). In the second set of experiments, 7-NI (50, 100 mg/kg) or an equal volume of vehicle was administered after unilateral carotid artery ligation, but 30 min before hypoxia in PND 7 rats. 7-NI (100 mg/kg) significantly protected against cerebral hypoxic-ischemic injury (100 mg/kg of 7-NI, 1.7+/-1.0% damage; control, 8.7+/-1.6%,P<0.05). 7-NI administered 15 min after cerebral hypoxia-ischemia was not neuroprotective. The data suggest that the protective effect of 7-NI is dose dependent, and is related to the duration of suppressed NOS activity.

Delayed increase in neuronal nitric oxide synthase immunoreactivity in thalamus and other brain regions after hypoxic-ischemic injury in neonatal rats

We examined the response of neuronal nitric oxide synthase (nNOS)-containing CNS neurons in rats exposed to a unilateral hypoxic-ischemic insult at 7 days of age. Animals were sacrificed at several time points after the injury, up to and including 7 days (Postnatal Day 14). Brain regions ipsilateral to the injury (including cerebral cortex, caudate-putamen, and thalamus) exhibited delayed, focal increases in nNOS immunoreactivity. The increase in nNOS immunoreactive fiber staining was prominent in areas adjacent to severe neuronal damage, especially in the cortex and the thalamus, regions that are also heavily and focally injured in term human neonates with hypoxic-ischemic encephalopathy. In cerebral cortex, these increases occurred despite modest declines in nNOS catalytic activity and protein levels. Proliferation of surviving nNOS immunoreactive fibers highlights regions of selective vulnerability to hypoxic-ischemic insult in the neonatal brain and may also contribute to plasticity of neuronal circuitry during recovery.

Inhibition of nitric oxide synthesis following severe hypoxia-ischemia restores autoregulation of cerebral blood flow in newborn lambs

Birth asphyxia impairs the autoregulatory ability of the cerebral blood flow. Inappropriate synthesis of vasodilatory nitric oxide may be important in this respect. We investigated if nitric oxide synthesis inhibition by N(omega)-nitro-L-arginine (NLA) could restore cerebral autoregulation after severe hypoxia-ischemia (HI). HI was induced in 15 newborn lambs. Cerebral blood flow (carotid artery blood flow [ml/min]: Qcar) and mean aortic blood pressure [mmHg]: MABP) were measured over a 30 min period before HI (pre-HI), 0-30 min after completion of HI (0-30 post-HI) and from 60 to 120 min post-HI (60-120 post-HI). Immediately after completion of HI, 5 lambs received a placebo (PLAC), 5 low dose NLA (10 mg/kg/iv: NLA-10) and 5 high dose NLA (40 mg/kg/iv: NLA-40). Pre-HI, all groups showed cerebral autoregulation with an upper limit of regulatory ability between 75 and 90 mm Hg. At 0-30 post-HI, all groups lacked autoregulatory ability of the cerebral vascular bed and showed an aortic blood pressure-passive Q(car). At 60-120 post-HI autoregulation was restored in NLA-10 and NLA-40-treated lambs (upper limit of autoregulation was shifted to higher MABP in NLA40-treated lambs), but not in placebo-treated lambs. At 60-120 post-HI MABP was higher in both NLA-groups than in PLAC group (83+/-15 [NLA-10] and 78+/-14 [NLA-40] vs. 65+/-9 mmHg [PLAC], P<0.05). We conclude that severe HI in newborn lambs induces impairment of the autoregulatory ability of the cerebral vascular bed. Even low-dose nitric oxide-synthesis inhibition started upon reperfusion restored autoregulation, suggesting a role for nitric oxide-induced vasodilation in the impairment of autoregulation of the cerebral blood flow after birth asphyxia.

Stroke: development, prevention and treatment with peptidase inhibitors

Stroke is the leading cause of long-term disability in the United States and affects more people than any other neurologic disorder. Hypertension is a major risk factor associated with stroke. Several anti-hypertensive agents have been used to treat chronic hypertension to reduce the morbidity and mortality of stroke. Previous experimental studies have shown reduced stroke mortality with angiotensin-converting enzyme (ACE) inhibitors. This review discusses the development of stroke and potential use of ACE inhibitors in prevention and treatment of this disease. Furthermore, this review focuses on current investigations aimed at cellular mechanisms involved in stroke-induced microvascular alterations.

7-Nitroindazole reduces nitric oxide concentration in rat hippocampus after transient forebrain ischemia

We investigated the effects of 7-nitroindazole, a specific inhibitor of neuronal nitric oxide (NO) synthase, on NO concentration and on blood flow in rat hippocampus after transient forebrain ischemia which was induced by 4-vessel occlusion for 10 min. NO concentration was measured directly by an NO-selective electrode method. Hippocampal blood flow was also estimated by laser Doppler flowmetry. 7-Nitroindazole [0 (vehicle), 12.5, 25, 50 or 100 mg/kg] was administered intraperitoneally 20 min before ischemia. 7-Nitroindazole at any dose used did not affect basal NO levels before ischemia. 7-Nitroindazole (25, 50 and 100 mg/kg) reduced the NO concentration significantly during post-ischemic early reperfusion. Before 10 min of ischemia and during post-ischemic early reperfusion, there were no significant differences in hippocampal basal blood flow and reactive hyperemia between vehicle- and 7-nitroindazole-treated groups. These results demonstrate that the neuronal NO synthase inhibitor, 7-nitroindazole, can effectively inhibit NO synthesis in rat hippocampus during post-ischemic early reperfusion.